The immobilization antigen (i-antigen) is a surface protein of the protozoan P. aurelia. In the presence of homologous antisera made against the i-antigen, the animals become immobilized and eventually die. Although the exact function of this protein is unknown, the i-antigen is of interest due to the unusual mechanism controlling its expression. Only one of as many as 12 i-antigens is believed to be produced by any one animal at any given time and this synthesis is affected by the state of the cytoplasm and the environment of the animal. Some knowledge of the i-antigen itself and a comparative analysis of the various i-antigens, will greatly aid in the understanding of the mechanisms involved in inter- and intra-cellular differentiation. A controversy as to the exact quaternary structure and make-up of the i-antigen has arisen because the methods previously used to purify the i-antigen have been shown to yield contaminating proteins, some of which contain degradative activity. We have used ion exchange chromatography on DEAE-Sephadex and SE-Sephadex and affinity chromatography (with specific antibodies attached to Sepharose) to purify the i-antigen. This purified preparation will be used to physically and chemically characterize the i-antigen.